The present invention relates to a liner for a flexible container and to such container incorporating a liner, particularly but not exclusively for transport and storage of bulk material, especially pulverized or granular material.
Flexible containers for bulk materials, so called FIBC's, normally comprise an outer container bag with lifting loops and a filling opening.
For most users of such flexible containers it is necessary that the material to be transported is protected against being contaminated by dust, water, etc., and this is usually achieved by the use of an impervious liner made from plastic sheet material, paper or suchlike which is positioned within the outer container, usually against the inner wall thereof, the liner being filled with the material to be transported and/or stored.
It has been found desirable to position the liner in its load-carrying outer container at the production site of the container, and one way of making such a composite container is to fasten the liner to the material forming the outer container, part way through the assembly of the container, with glue or tape, thereafter finishing the sewing of the outer container.
However it has proved to be difficult to place the liner in the outer container in such a way that it is neither damaged, nor crumpled and adopts a form which matches that of the outer container. Otherwise the liner becomes prone to bursting when filled with bulk material. Furthermore, displacement and unfastening of the liner may occur during use and obviously placing the liner correctly during bag manufacture does not help if it is displaced before or during the filling operation.
one method of aligning the liner in the outer container is disclosed in European Patent 0141429 in which the container is placed inside a folding means in the form of a tunnel or open ended box having a square cross-section, the liner thereafter being placed inside the finished outer container and then inflated. The longitudinal side walls of the box are divided and joined together, for example, by hinges. Subsequent to inflation the box is pressed together and during this operation the hinged side walls form gussets in both the liner and the outer container. The container and liner are pressed together and can then be drawn out of the box and, optionally can also be folded crosswise of the longitudinal direction, and are then ready for transport to the filling site for bulk material.
Due to the open nature of the woven cloth making up the outer container, all superfluous air between the outer container and the inner liner will be squeezed out so that in the final flattened form no air is present between the liner and the outer container, thus avoiding difficulties at later filling stages and ensuring that the liner and the outer bag are in correct alignment.
This method, while providing a correct, smooth and crease-free fit between the inner liner and the outer bag, does, however, require an extra manufacturing step at the bag production site. Furthermore, the method does not solve a further problem caused by air remaining inside the inflated liner that is entrapped by the granular, particulate bulk material during filling of the bags. Such air remaining inside the liner bag after the bag is filled may constitute as much as up to 10 to 15% of the total volume of the bulk material, when the filling spout of the liner is sealed, which is desirable to obtain maximum protection. The trapped air will only leak out gradually over time and causes considerable problems in connection with the subsequent handling, transport and storage of the filled bags.
When the filled sacks are handled and stacked, the contents settle and compact, allowing the trapped air to form a deformable cushion within the sack. This can cause the sack to adopt a rounded shape, affecting the stability of a stack of the sacks and making it difficult to handle the filled sacks mechanically. Furthermore, the trapped air will expand or contract with variations in the surrounding temperature, causing the air cushion to swell and deflate. In an extreme case, this can cause the wall of the sack to rupture where excess expansion of the air cushion takes place.